Patent GB-823541-A (GB)
Title: Improvements in or relating to secrecy television systems
IPC: H04N7/16
Filed: 1956-01-12
Abstract: 823,541. Television. INTERNATIONAL TELEMETER CORPORATION. Jan. 12, 1956 [June 7, 1955], No. 1092/56. Class 40 (3). A subscription television system of the kind in which, in advance of viewing special transmissions which have been rendered unintelligible by a coding arrangement, subscriber receivers (i.e. those fitted with the necessary decoding apparatus) must satisfy a coin demand by depositing the necessary coinage in a coin-box (forming part of the decoding apparatus) which then allows the transmissions to be rendered intelligible, comprises means for transmitting simultaneously two sets of video and associated audio signals a first set, termed " marquee " video and "barker" sound, giving visual and oral details of the special transmission before the coin demand is satisfied and a second set, termed " programme " video and " programme " sound, conveying the special transmission in unintelligible form until satisfaction of the coin demand when it is rendered intelligible and replaces the first set of signals as reproduced by subscriber receivers. In the composite signal " programme " video is transmitted as amplitude modulation of a sub-carrier wave at one or other of two predetermined phases the change from one phase to the other being effected at random intervals to produce the required coding and the frequency of the sub-carrier being selected with reference to the line scanning rate of the system and to a main carrier wave on which the " marquee " video signals are modulated to produce frequency interleaving in similar manner to known colour television arrangements in which luminance information is transmitted on a main carrier wave and chrominance information on a subcarrier wave. A receiver tuned to such a transmission (in the absence of de-coding) will display the " marquee " video but the " programme " video will not be visible due to the integration effect of the eye producing cancellation. As shown in Fig. 6 such a composite signal is derived by supplying the output of a modulator 612, in which the " programme " video signals from camera A are modulated on to phases #1 or #2 of a sub-carrier wave from generator 608 the phases being selected at random under the control of a coder 622, via a filter 614 and gate 623 to an adding circuit 602 in which it is combined with the " marquee " video signals from camera B the combined output being fed to a transmitter 625. During retrace intervals the gate 623 is operated under the control of signals from the synchronising generator 606 to interrupt the signals from modulator 614 and to supply 1 instead to adder 602 the output of a modulator 624 which, during retrace intervals, comprises either a 5 Ásec. " burst " of subcarrier (Fig. 2) or a 1.5 Ásec. " burst " of a frequency equal to <1>/3 the sub-carrier frequency followed by a 3.5 Ásec. " burst " of sub-carrier (Fig. 3) which of the two signals occurs being determined by the coder 622 and indicative of the particular phase #1 or #2 then employed in modulator 612. At a subscriber receiver (Fig. 13, not shown) the frequency which occurs during the first 1.5 Ásec. of the interval is utilized to control the decoding action by switching in or out delay devices introducing similar phase changes the actual change (at both transmitter and receiver, occurring during the next vertical blanking interval and the remainder of the " burst " employed both for the control of the receiver sub-carrier generator and for normal horizontal synchronization. For the transmission of audio signals two channels A and B (Figs. 7A to 7D) are employed, channel A, the " normal " sound channel, extending from 0 to 15 kc/s. and channel B from 16.5 kc/s. to 31.5 kc/s. Both of these channels are carried by a normal F.M. sound transmitter. The " programme " and " barker " sound signals respectively, are carried by each channel, alternately, the change-over being controlled by a random signal generator which initiates a slow fade-out of one sound signal and a slow fade-in of the other on each channel the actual change-over occurring during an overlap period when " programme" sound is carried by both channels to prevent any possible interruption of the latter. At a subscriber receiver (Fig. 13, not shown) automatic following of the " programme " sound as shown in Fig. 7c is effected when the coin demand has been met by conditioning the audio circuits to respond to the correct channel under the control of decoding signals. If the coin demand has not been met, " barker " sound only with occasional bursts of " programme " sound during the overlap period are received as shown in Fig. 7D whilst a non- subscriber receiver only receive channel A consisting of " barker " and " programme " sound alternating in random manner (Fig. 7E). In order that non-subscribers would know that their receivers were functioning correctly and that the sound is being deliberately rendered unintelligible a long " barker " sound signal is periodically transmitted on channel A. For the control of the video and audio decoding a code consisting of 16 binary digits is transmitted during 16 line blanking intervals. Of these 16 digits ten form an identification number, three control the video decoding and three the audio decoding. In addition to the latter, coin-box control signals are also transmitted. These signals control (1) a price wheel device in subscriber receivers to indicate the amount of coinage that must be inserted in the coin-box in order to effect decoding of the programme; (2) control a recording device which indicates the programme purchased; and (3) control the operation of a selective switching arrangement which allows the decoding circuits to respond to the code signals for that programme. The Specification includes a detailed description of the generation, transmission and utilization of all the code and control signals.